We present new experimental measurements of rotationally inelastic scattering of vibrationally excited NO(X2Π) with Ar and CH4. A molecular beam of NO was prepared in a single rotational and parity-resolved state, j = 1.5 F1e, in the v = 1 vibrational level using mid-infrared radiation from a distributed feedback quantum cascade laser. Following collision with a crossed molecular beam of Ar or CH4, rotationally excited NO(X, v = 1) in the isolated final rotational states j' = 4.5 F1f and j' = 10.5 F1f was detected by 1 + 1' resonance-enhanced multiphoton ionization coupled with velocity-map imaging. Differential cross sections and rotational angular momentum polarization moments for inelastic scattering with Ar are in excellent, near-quantitative agreement with quantum scattering predictions on a literature potential energy surface. Images for scattering from CH4 for both final states show clear evidence of significant rotational excitation in the CH4. Overall, a negative correlation is observed in the NO-CH4 rotational excitation, with higher average CH4 rotational energy for final NO j' = 4.5 than for j' = 10.5. For NO j' = 10.5, higher rotational energies of CH4 are surprisingly correlated with forward hemisphere scattering, while lower CH4 rotation is correlated with backward hemisphere scattering. These measurements demonstrate the importance of the preparation of an initial rotational and parity-selected state, and the varied and surprising dynamics that remain underexplored in molecule-molecule inelastic scattering.
Fournier et al. (Mon,) studied this question.